The present invention relates to vehicle heaters that shear viscous fluid to generate heat and transmit the heat to a coolant fluid. More particularly, the present invention relates to a viscous fluid heater employing a rotor having an inclined shearing surface.
Viscous fluid heaters are used as an auxiliary heat source for automobiles and are driven by the force of the engine. Japanese Unexamined Patent Publication No. 2-246823 describes a typical viscous fluid heater, which is incorporated in an automobile heater.
The viscous heater has a front housing element and a rear housing element that are coupled to each other to form a housing. A heating chamber and a water jacket (heat exchange chamber), which encompasses the heating chamber, are defined in the housing. A drive shaft extends through the front housing element and is rotatably supported by a bearing. A rotor is fixed to one end of the drive shaft in the heating chamber so that the rotor and the drive shaft rotate integrally with each other. Walls project axially from the front and rear surfaces of the rotor. Grooves are defined in the heating chamber walls to receive the rotor walls. A clearance is provided between the rotor walls and the heating chamber grooves. The clearance contains a predetermined amount of viscous fluid such as silicone oil.
When engine power is transmitted to the drive shaft, the rotor is rotated integrally with the drive shaft in the heating chamber. This shears the viscous fluid located between the rotor surface and the heating chamber walls. The shearing effect causes fluid friction that generates heat. The heated silicone oil exchanges heat with engine coolant, which circulates through the water jacket. The heated coolant is then sent to an external heater circuit and used to warm the passenger compartment.
In the prior art heater, the viscous fluid is constantly sheared by the rotor. Furthermore, the rotating velocity of the rotor (shearing velocity) is higher at positions located farther from the axis of the rotor. Thus, the shearing velocity is higher at the periphery of the rotor. This may result in local overheating of the viscous fluid located near the periphery. Such overheating leads to early deterioration of the viscous fluid.